The plant vascular system is responsible for transporting water, nutrients and photosynthates between plant organs. It also undergoes developmental adaptations such as wood formation, which involve specific proliferation of the vascular tissue. Therefore, the pattern of cell divisions is an important determinant of the cellular organization of this tissue (Esau, 1977, Anatomy of seed Plants. John Wiley & Sons, New York, N.Y., ed. 2.). Vascular tissue is first established during embryogenesis as an undifferentiated procambial tissue in the innermost domain of the plant embryo, enclosed by the epidermal and ground tissue layers (Esau 1977 supra; Steeves & Sussex, 1989, Patterns in Plant Development. Cambridge University Press, Cambridge, UK). After differentiation of the phloem and xylem strands within this domain, cell proliferation originates primarily from the initial cells of the procambial tissue immediately proximal to the mitotically quiescent regions of the terminal meristems (Esau 1977, supra; Steeves and Sussex 1989, supra; Scheres et al. 1994, Development 120:2475–87). Later in development, a lateral meristem (the cambium) is formed, as the undifferentiated cells begin to divide in the procambial tissue between the phloem and xylem strands. There is a high degree of diversity of the cell division patterns within the vascular tissue in plants, especially with regards to the formation and activity of the cambium. Since these patterns are species-specific, it is conceivable that the control of cell proliferation within the vascular tissue is largely under genetic regulation.
Several factors have been implicated in the regulation of cell proliferation of the vascular tissue. Based on mutation analyses, signal transduction pathways related to auxin (Carland & McHale, 1996, Development 122:1811–9; Oyama et al., 1997, Genes Dev. 11:2983–95; Hardtke & Berleth, 1998, EMBO J. 17:1405–11; Hobbie et al., Development 127:23–32; Steinmann et al., 1999, Science 286:316–8; Koizumi et al., 2000, Development 127:3197–204) and brassinosteroid (Schrick et al., 2000, Genes Dev. 14:1471–84; Jang et al., 2000, Genes Dev. 14:1485–97) phytohormones are involved. Physiological and genetic experiments have also indicated a role for other phytohormones (such as gibberellins, cytokinins and ethylene; see Aloni, 1987, Annu. Rev. Plant Physiol. 38:179–204; Eriksson et al., 2000, Nat Biotechnol. 7:784–8), sucrose (Warren Wilson, 1978, Proc. Roy. Soc. London Series B 203:153–76) and physical pressure (Zimmerman, 1964, The Formation of Wood in Forest Trees. Academic Press, New York, N.Y. pp. 389–404). Furthermore, a few genetic loci have been identified that are essential for normal cell proliferation but function in a yet uncharacterized molecular context (Carland et al., 1999, Plant Cell 11:2123–37; Scheres et al., 1995, Development 121:53–62).
Root organization is established during embryogenesis. This organization is propagated during postembryonic development by the root meristem. Following germination, the development of the postembryonic root is a continuous process, a series of initials or stem cells continuously divide to perpetuate the pattern established in the embryonic root (Steeves & Sussex, 1972, Patterns in Plant Development, Englewood Cliffs, N.J.: Prentice-Hall, Inc.).
Due to the organization of the Arabidopsis root, it is possible to follow the fate of cells from the meristem to maturity and identify the progenitors of each cell type (Dolan et al., 1993, Development 119:71–84). The Arabidopsis root is a relatively simple and well characterized organ. The radial organization of the mature tissues in the Arabidopsis root has been likened to tree rings with the epidermis, cortex, endodermis and pericycle forming radially symmetric cell layers that surround the vascular cylinder (Dolan et al., 1993, Development 119:71–84). These mature tissues are derived from four sets of stem cells or initials: i) the columella root cap initial; ii) the pericycle/vascular initial; iii) the epidermal/lateral root cap initial; and iv) the cortex/endodermal initial (Dolan et al., 1993, supra). It has been shown that these initials undergo asymmetric divisions (Scheres et al., 1995, Development 121:53–62). The cortex/endodermal initial, for example, first divides anticlinally (in a transverse orientation). This asymmetric division produces another initial and a daughter cell. The daughter cell, in turn, expands and then divides periclinally (in the longitudinal orientation). This second asymmetric division produces the progenitors of the endodermis and the cortex cell lineages.
Citation or identification of any reference herein shall not be construed as an admission that such reference is available as prior art to the present invention.